Multi-stage gain amplifiers are widely utilized in telecommunications and other applications where signals are transmitted and received. The applications can include both wireless and wired transmission and reception of signals.
Research and development in telecommunications applications have been expanding to meet the increasing popularity in the Internet. One particular application relates to digital subscriber line (DSL) technologies, including both symmetric DSL (SDSL) and asymmetric DSL (ADSL). ADSL is a modem technology that converts existing telephone lines into access paths for multimedia and high speed data communications. ADSL utilizes advanced digital signal processing and creative algorithms to provide greater amounts of information through twisted-pair telephone lines than was conventionally feasible. The ADSL standard calls for a specifically designed modem at each end of a twisted pair copper line, one at a home and the other at a central office of the phone company. Although the conventional telephone voice circuit has only a 4 KHz bandwidth, the physical wire connection bandwidth exceeds 1 MHz.
ADSL exploits the extra bandwidth to send data to the central office where it can connect to a phone company's high capacity fiber optic network. ADSL modems increase the amount of information that conventional phone lines can carry buy using discrete multi-tone technology (DMT). DMT divides the bandwidth into independent subbands, then transmits data on all of the subbands simultaneously. Echo cancellation techniques allow upstream and downstream data to overlap. ADSL is particularly attractive for consumer Internet applications where most of the data traffic is downloaded to the customer. Upstream bandwidth for uploading data can be reduced to increase downstream bandwidth since most Internet traffic is downstream.
Terminals employed at the central office typically communicate over several channels. These terminals employ modems or other communication devices for data transmissions. These modems or other communication devices can employ a digital processor, a coder/decoder component, line drivers and other peripheral devices to support transmitting and receiving of analog signal transmissions. The central office line driver drives the ADSL signal onto telephone lines. The ADSL signal includes data streams that are carried over tones that are amplitude, phase modulated and frequency separated by about 4 KHz. Conventional ADSL modem designs include functions referred to as analog front ends in which operations, such as digital-to-analog and analog-to-digital conversion, amplification/attenuation and filtering, are performed. Because of the frequencies involved in ADSL technology, which can range from tens of KHz to MHz frequencies, and because of the large dynamic range required in order to accommodate the wide variations in length and schemes for subscriber loops, the amplification and filtering can be very complex, particularly in an integrated circuit.
Analog amplification or attenuation is typically needed in the received path of most digital modems to fully utilize the available digital dynamic range. Typically, the gain adjustment depends on the particular channel conditions and loop characteristics. Therefore, automatic gain control is preferable. When the PGA and filtering stages are interleaved, as in typical integrated analog front ends, the automatic gain control process becomes more complicated since, in most environments, the intermediate points in the receive path are inaccessible.